With printing presses, the exact alignment of the conveyed printing material on a conveyor belt is of considerable importance for the desired application of the printing format in the correct position on the printing material. Sheets of printing material may be conveyed on a conveyor belt through the printing press, whereby misalignments of the sheets transverse to the conveying direction are corrected by sheet-grasping conveyor rollers that solve the problem (see for example U.S. Pat. No. 5,322,273). Furthermore, sheets are moved as desired before being deposited on a stack following the printing process, in order to arrange the sheets so that they are offset on the stack. The printing press operator can subsequently remove the sheets in staggered piles without having to sort them.
The conveyor rollers are attached to a shaft, unroll on the printing material and are moved by a spindle in the shaft transverse to the conveying direction corresponding to a calculated transverse movement of the sheets. Due to the friction force of the conveyor rollers on the sheets, the sheets are transversely moved according to the transverse movement of the conveyor rollers. The spindle, which is moved in the conveying direction of the sheets, can be driven by a belt drive of a stepping motor. The stepping motor receives corresponding control signals from a control device of the printing press, which are adjusted to the dimensions of the calculated dimensions of the misalignment to be corrected or the desired offset of the sheets on the conveyor belt. The stepping motor is connected to a belt drive, which converts the rotating movement of the stepping motor into movement of the spindle. It must be ensured that that the spindle in the shaft, to which the conveyor rollers are attached, does not exceed a certain range of travel. If the range of travel is continuously exceeded by the spindle and therefore by the shaft, this may lead to damages of the spindle, the shaft, as well as to the shaft housing, and to the failure of the device for offsetting the sheets.
A solution to the described problem is found in adjusting the drive of the stepping motor to the maximum permissible range of travel of the spindle, i.e., when the stepping motor has carried out a certain number of steps in one direction, the stepping motor is stopped. However, this solution is only permanently suitable if the driving of the stepping motor can be carried out with high reliability. The stepping motor is driven by software in the control device of the printing press. If the software fails, which happens frequently, the accurate driving and the reduction of the spindle movement at a maximum permissible range of travel is not guaranteed. A further solution to the existing problem mentioned is the use of light barriers, which monitor the end positions of the range of travel and signal the control device. Failure of the light barriers, however, equally leads to the failure of the protection before the permissible range of travel is exceeded, and thus does not represent any suitable solution.